Sudbury basin ores, like many other sulphide deposits, contain pyrrhotite which, having little or no commercial value, may be regarded as a sulphide gangue. Sudbury ores comprise in an increasing order of abundance: chalcopyrite (Cp), pyrite (Py), pentlandits (Pn), and nickeliferous pyrrhotite (Po) as the principal sulphides along with some other sulphides in small and variable amounts. Non-sulphide gangue minerals consist of mainly quartz and feldspar along with minor quantities of tremolite, biotite, magnetite and talc. Pyrrhotite which typically represents between 20 and 25% of the ors, is intimately associated with other minerals, primarily with pentlandits. In the treatment of such complex ores, some process streams may consist essentially of all pentlandite-pyrrhotite middlings containing more than 70% pyrrhotite. These streams have always presented a serious separation problem. Most of the complex sulphide ores of different mineralogy have similar separation problems. Poor separations result in low concentrate grades of valuable minerals. The presence of iron sulphides in the concentrates of non-ferrous base metals is almost always undesirable. In the processing of nickel-copper ores in the Sudbury region, a selective separation process will allow an economical rejection of the least valuable sulphide component, pyrrhotite which is the main contributor to sulphur dioxide emissions from smelters.
Pyrrhotite is separated from its associated minerals using a process of magnetic separation or flotation. The field of present invention is the latter. In general, the flotation process involves the grinding of the crushed ore in a dense slurry to the liberation size, followed by conditioning with reagents in a suitably dilute slurry. Broadly, reagents may function as collectors which determine the surface hydrophobicity (aerophilicity) of minerals, frothers which generate stable bubbles of suitable sizes in slurry for the capture and transfer of particles to the froth phase for their removal as concentrate, depressants which have the reverse action to collectors causing the surfaces of selected mineral particles to become hydrophilic thus allowing their rejection to tails. Flotation may be carried out as a single stage or in multiple stages.
The present invention describes a process for depressing iron sulphides and more specifically pyrrhotite and nickeliferous pyrrhotite during the flotation of nickel and other valuable base metal sulphides. It is of the utmost importance that any depressant used in a commercial operation be consistently effective and, while a variety of reagents are recognized as having selective function in the flotation of minerals containing various base metals, their action alone has been found to be unpredictable on pyrrhotite. Diethylenetriamine (DETA) is one of the preferred reagents employed for the purpose of the current invention. The depressant action of DETA in sulphide mineral beneficiation is known in the art. This is a reagent common to three U.S. patents issued to Griffith et al (U.S. Pat. No. 4,139,455), Bulatovic et al (U.S. Pat. No. 4,877,517) and Kerr et al (U.S. Pat. No. 5,074,993).
DETA (H.sub.2 N--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CH.sub.2 --NH.sub.2) belongs to a family of polyamines with a general technical name "[n] ethylene [n+1] amine" representing a series of relatively simple ligands. An ethyleneamine unit is added into molecular structure to form a homologous series. The simplest member of the family is monoethylenediamine (n=1), which is designated in chemical literature by its short version as "en". Similarly, diethylenetriamine (DETA) is commonly known by its short form as "dien" (i.e., n=2), triethylenetetramine as "trien" (i.e., n=3). These polyamines do not have any tertiary amine group in their structure.
The polyethylenepolyamine depressants, exemplified in the current process by DETA, differ from the iron sulphide depressants described by Griffith et al (U.S. Pat. Nos. 4,078,993 and 4,139,455) and by Bulatovic et al (e.g., U.S. Pat. No. 4,877,517) in that the latter are essentially the reaction products of several additional reagents such as formaldehyde, adipic acid, caustisized starch, polyacrylic acid etcetera. The process disclosed by Griffith et al. also requires a tertiary amine group to be present in the depressant structure. The resulting polymeric structures are viscous, having rather large molecules in which the nitrogen atom is a link in the polymer chain structure.
U.S. Pat. No. 5,074,993 to Kerr et al., issued on Dec. 24, 1991, describes the use of water-soluble polyamines as a pyrrhotite depressant for the selective flotation of nickel-copper minerals. The success of the process is demonstrated by various examples, using feed samples in which Po/Pn ratio is relatively low, with one exception (at 15) lower than 10. The process behaviour of pyrrhotite-rich streams is not necessarily the same as those containing relatively low pyrrhotite content. As those skilled in the art would readily agree, the difficulty in Pn-Po separation by selective flotation of pentlandits from pyrrhotite increases with an increase in Po/Pn ratio of the feed to a specific flotation stage. Accordingly, a different set of conditions is usually required to meet the special demands of the processes intended for difficult-to-treat complex sulphides. As will be noted in the examples to follow, the depression effect on pyrrhotite of DETA by itself is unacceptably poor in the treatment of Po-rich process middlings.
The current invention differs from the process described by Kerr et al (U.S. Pat. No. 5,074,993) as well as those by Griffith et al and Bulatovic et al (already cited hereinbefore) in that it provides a specific conditioning stage with sulphur-containing auxiliary reagents. In the patent to Kerr et al, the NCCN configuration of said polyamines is emphasized as a specific requirement for the depression effect on pyrrhotite, an observation that also differs from that provided in the current disclosure.
One of the reagents tested is histidine which has the following structural formula: ##STR1## It has a primary amine group attached to ethylene chain which in turn is attached from one end to a five-membered ring containing two nitrogen atoms as in tertiary and secondary amines, respectively. For the purpose of comparison in terms of atomic arrangement, this molecular structure may be viewed as OCNCCCNCNC or alternatively, OCNCCCCNCN owing to the ring moiety. As will be noted from the results in specific examples, this structure is also capable of depressing pyrrhotite in preference to pentlandits. However, the depressant function induced by both this configuration and the NCCN configuration in DETA structure is dependent on an essential process stage which constitutes the essence of the current invention.